Code:
on *:TEXT:*:#: {
  if (hello isin $1-) {
    set %rndnmbr $rand(0.4)
    if (%rndnmbr == 0) { set %rslt Greetings }
    elseif (%rndnmbr == 1) { set %rslt Hey }
    elseif (%rndnmbr == 2) { set %rslt Hi }
    elseif (%rndnmbr == 3) { set %rslt What's up }
    elseif (%rndnmbr == 4) { set %rslt Sup }
    msg $chan %rslt $nick
  }
}


This is untested, but I'm sure someone else will have a better code, or at least tweak this code for you.

I registered; you should too.